Breakthroughs in many of today's cutting-edge technologies are becoming increasingly dependent upon the ability to safely dissipate enormous amounts of heat from very small areas. A key measure of heat dissipation density is heat flux, or heat removal rate divided by heat dissipation area. Two ranges of device heat fluxes can be identified relative to the magnitude of the heat dissipation and the type of coolant permissible in a particular application. These are the high-flux range, with heat flux requirements on the range of 100 to 1000 watts per square centimeter (W/cm2), and the ultra-high-flux range, with heat fluxes exceeding. High-flux cooling systems are presently needed in many electronic and power applications, such as personal computers, mainframe computers, avionics (i.e., aviation electronics), fuel cells, and satellite electronics. Ultra-high-flux cooling systems, on the other hand, are important to defense lasers, microwave weapons, radars, x-ray medical devices, nuclear fusion reactors, rocket nozzles, and turbine blades.
Achieving reliable cooling for high-flux and ultra-high-flux applications typically requires pumping a liquid coolant at enormous flow rates through intricate cooling passages situated close to the heat-dissipating surface. Such coolant flows can produce extreme pressure drops, which necessitate the use of specialized powerful pumps. The challenge in cooling emerging and future devices and systems is to remove enormous amounts of heat using compact, lightweight cooling hardware, low coolant flow rates and relatively low pressure drops. This is a formidable challenge considering that such coolant conditions are typically incompatible with high-flux removal.
Heat removal from electrical-current-carrying devices poses many unique challenges. High performance computer chips are a prime example of an application that is constantly demanding new, aggressive cooling solutions. Despite the many recent signal speed-enhancing breakthroughs in such areas as superconductivity, optical transmission, gallium arsenide semiconductors, and parallel processing, the ever increasing demand for greater signal speed has created major challenges to the manufacturers of both commercial and defense electronics. These challenges are difficult to meet even with today's most cutting-edge cooling technologies.
In fact, heat removal has emerged as an obstacle to electronic packaging at all levels: device, multi-device module, and system. One reason for this trend is the alarming increase in the number of electronic components that are being packaged into the device itself (e.g., computer processor) through aggressive micro-miniaturization. Thermal management obstacles have also been compounded by the increased signal speed requiring shorter distances between devices in multi-device modules and a closer packaging of modules in a cooling system.
Air cooling is still widely used and will always be favored, where possible, over all other cooling techniques; but the poor thermal transport properties of air require forced circulation at speeds that often exceed upper accepted limits for noise and vibration. To avoid exceeding those limits, many high-power devices can be found mounted on massive fins—heat sinks—which are intended to greatly increase the surface area available for air cooling. Even with the finning and the increased air speed, air is not likely to handle the high heat fluxes projected for the next generation of devices while maintaining acceptable device temperatures. For this reason, new cooling technologies have emerged in recent years that rely upon direct or indirect cooling with liquids.
Different types of liquids can be used in high-flux and ultra-high-flux cooling systems. These include water, fluorochemicals, glycols, oils and liquid metals. Fluorochemical liquids have received considerable attention for electronic cooling applications because their cooling performance is vastly superior to that of air. Furthermore, many of these coolant possess excellent dielectric properties; these liquids can be used in direct contact with an electronic or electrical device without short-circuiting components in the device itself or the connector board. However, the cooling effectiveness of fluorochemicals is quite inferior to that water, which explains why many high-flux and ultra-high-flux devices are cooled with water.
Aside from the type of liquid coolant used, cooling effectiveness can be greatly improved by pumping the coolant to the heat-dissipating device surface at high speed, and more importantly, by allowing the coolant to undergo a change-of-phase, from liquid to vapor, i.e., by boiling upon the device surface.
For liquid-cooled devices, when the device heat dissipation level is relatively small, the heat is removed by heat transfer to the liquid without boiling. This form of heat transfer is associated with a fairly linear increase of device temperature with increasing device heat dissipation rate. Boiling commences on the device surface after the device temperature exceeds the liquid's boiling temperature. The growth and departure of vapor bubbles on the device surface draws liquid toward the device surface at high frequency which, along with the ensuing latent heat exchange resulting from the vapor production, greatly increase cooling effectiveness. Any subsequent increase in the device heat dissipation rate is removed with only a modest increase in the device temperature. Thus, the device temperature is much lower in the presence of boiling than with pure liquid cooling.
The attractive cooling performance associated with boiling is realized in the so-called nucleate boiling regime. Cooling effectiveness within this regime requires uninterrupted liquid access to the device surface to replace the vapor released in the form of bubbles. Higher heat dissipation rates are removed by the production of more vapor bubbles per unit surface area. While this increased vapor bubble formation is beneficial to the heat removal process, it can also produce appreciable bubble crowding and bubble coalescence upon the device surface. As the heat dissipation rate increases further, the bubble crowding and coalescence begin to restrict liquid access to the device surface. A condition is eventually reached when the liquid access is interrupted, and heat that is dissipated by the device itself can longer be removed by the liquid—the device temperature begins to escalate uncontrollably. This condition is the upper limit for the nucleate boiling regime and is termed critical heat flux (CHF). CHF constitutes the upper heat dissipation design limit for most cooling systems that employ boiling.
Airborne and space-based electronic systems pose packaging constraints that are far more stringent than those of even mainframe and supercomputers. Due to severe weight and volume limitations, the electronics in these systems must be configured using compact, lightweight two-dimensional and three-dimensional architectures. Close proximity between devices, in addition to the aforementioned high concentration of integrated circuits in the device itself, has led to enormous heat dissipation rates in several aerospace and defense systems.
One important application of defense electronics is high power lasers. These lasers are used for targeting, tracking, and in communications systems. Unfortunately, lasers are thermally inefficient, i.e., enormous amounts of heat are needlessly dissipated and have to be safely removed to maintain system integrity. The heat fluxes associated with these heat dissipation systems greatly exceed those encountered in even the most powerful commercial lasers and therefore require high performance liquid cooling schemes.
A key concern in the implementation of a cooling scheme in defense applications is the large differences in operating environment for different defense platforms. Each platform (ground, air, sea or space) imposes stringent requirements concerning such important parameters as weight, volume, shock, vibration, ambient temperature, and G-force. The latter is of particular concern to military aircraft whose maneuvers create large forces that complicate liquid flow inside cooling modules, especially where vapor is present due to boiling.
Numerous cooling schemes have recently been developed for high-flux heat removal from electronic devices. Each scheme provides specific merits for a particular application. Unlike most recently developed cooling schemes, which seem to target cooling fluxes of about 100 W/cm2, the present invention concerns a cooling scheme that is capable of exceeding 1000 W/cm2 in order to serve electronic, aerospace and defense needs for decades to come.
High liquid coolant flow rates are often needed to meet high-heat-flux cooling requirements, and such flow rates are typically associated with extreme pressure drops, especially when implemented with boiling. However, many of the applications for which the present invention are intended do not use massive liquid pumps or complicated coolant conditioning loops. What is therefore needed for these applications is a cooling system that is capable of meeting the intense cooling demands, but requires low flow rates and low to moderate pressure drops.
Due to the liquid motion, significant bubble coalescence can occur over a multi-device module as illustrated in FIG. 1A. This is especially the case where a saturated liquid coolant 1 is used, i.e., where the liquid is supplied at the boiling temperature. As illustrated in FIG. 1A, gradual stream-wise bubble coalescence can result in appreciable thickening of a vapor layer 2 along the surfaces of the devices 3. This phenomenon can trigger CHF prematurely on downstream devices.
These problems can be alleviated by subcooling the liquid, i.e., by supplying it at a temperature much lower than the boiling temperature. Subcooled flow is categorically different from saturated flow in all stages of the vapor layer development. As shown in FIG. 1B, a highly subcooled liquid 4 may result in complete condensation of vapor bubbles 2 back into liquid state only a short distance downstream of the device from which the bubbles are generated. Thus, subcooled flow takes full advantage of nucleate boiling while ensuring the coolant would exit the cooling module in liquid state.
The coolant exiting the cooling module is typically returned to the module at the desired temperature, pressure and flow rate with the aid of an external coolant conditioning loop. The presence of vapor at the module exit can complicate the operation of the conditioning loop. Thus, by completely condensing the vapor before exiting the module, subcooled flow would require a relatively simple liquid loop to condition the coolant external to the module.
Another important advantage of subcooled flow is that the rapid condensation of bubbles maintains liquid access to the device surface during intense heat removal. This a powerful means for delaying CHF, i.e., allowing larger heat removal rates than with saturated flow.
CHF is the upper design limit for most cooling systems that employ boiling. Exceeding this limit can result in permanent device damage or burnout. Adopting methods that delay (i.e., increase) CHF amounts to extending the nucleate boiling regime to accommodate higher heat removal rates.
Published studies point to (1) increasing liquid velocity, (2) increasing subcooling, and/or (3) surface enhancement (i.e., incorporating surface fins) as three effective methods for delaying CHF. However, delaying CHF is sometimes realized with cooling system penalties. For example, large liquid velocities require large flow rates and produce large pressure drops. These will both require a large pump and increase the size and weight of the coolant conditioning loop. As indicated above, practical considerations in many of the applications of the invention are intended for favor compact, lightweight components, hence the need to avoid large flow rates or excessive pressure drops.